I. Technical Field
The present invention relates to a surface emitting laser and a laser projector, and more particularly, to a surface emitting laser that operates stably at high output power, and a laser projector using the surface emitting laser as a light source.
II. Description of the Related Art
A surface emitting laser is a semiconductor laser that oscillates with a low threshold and has a superior beam quality, and applications of the surface emitting laser to an optical communication field are realized, utilizing the modulation characteristic that can modulate output light at a high speed. However, a problem with surface emitting lasers is the difficulty in realizing high output power.
Since a surface emitting laser is composed of thin films, a resonator length thereof is quite short because of its structure. Therefore, it is difficult to obtain a sufficient gain by increasing the resonator length.
On the other hand, high output power can be obtained by increasing a driving current of the surface emitting laser. However, when the carrier density in an active layer is too high in the surface emitting laser, light output is saturated due to gain saturation caused by spatial hole burning, and thereby high output operation is prevented.
Therefore, increasing the beam cross-section in the surface emitting laser is effective to achieve high output power of the surface emitting laser without increasing the resonator length or the driving current.
In the surface emitting laser having a short resonator length, however, when the beam cross-section is increased to increase the output power of the laser, the transverse mode in the resonator undesirably becomes multimode, leading to significant reductions in beam quality and oscillation efficiency.
Meanwhile, a surface emitting laser which prevents such reductions in beam quality and oscillation efficiency has already been developed. For example, U.S. Pat. No. 6,404,797 (the '797 patent discloses a surface emitting laser in which the beam cross-section is increased while preventing the transverse mode from becoming multimode.
FIG. 14 is a diagram for explaining the surface emitting laser disclosed in the '797 patent, and specifically, FIG. 14(a) is a cross-sectional view thereof, FIG. 14(b) shows the shape of a lower electrode, and FIG. 14(c) shows a light intensity distribution in a region of an active layer of this surface emitting laser, which region is opposed to the lower electrode.
The surface emitting laser 200 shown in FIG. 14(a) includes a semiconductor substrate 2, an active layer 3 disposed on one surface of the semiconductor substrate 2, and a reflection layer 4 disposed on the active layer 3. The reflection layer 4 is a distributed Bragg reflection layer obtained by alternately laminating materials 4a and 4b having different refractive indexes, and hereinafter, it is referred to also as a DBR layer. Further, the surface emitting laser 200 includes a circular lower electrode 600 disposed at the surface of the DBR layer 4, and an annular upper electrode 5 which is disposed on the other surface of the substrate 2 so as to surround the region opposed to the lower electrode 600.
The surface emitting laser 200 further includes an external mirror 1 which is disposed above the surface electrode 5 so as to be opposed to the surface of the substrate inside the surface electrode 5. In the surface emitting laser 200, a resonator for amplifying light generated in the active layer 3 to generate laser oscillation is constituted by the DBR layer 4 and the external mirror 1. The DBR layer 4 is a total reflection layer, and the external mirror 1 is a partial transmission mirror.
Next, the operation will be described.
In the surface emitting laser 200, when a driving voltage is applied between the upper electrode 5 and the lower electrode 600 and a current is injected into the active layer 3, light is generated in the active layer 3, and the generated light is amplified by the resonator. When the magnitude of the injected current exceeds a predetermined value (i.e., a laser oscillation threshold), laser oscillation occurs in the resonator, and laser light 8 is emitted through the external mirror 1 to the outside. At this time, the laser light 8 is surface-emitted, and the emission direction thereof is perpendicular to the surface of the substrate 2.
As described above, in the surface emitting laser disclosed in the '797 patent, a mirror as one of the components of the resonator is disposed separately from the substrate as the external mirror 1, thereby to increase the resonator length.
To be specific, since the external mirror 1 is used, the resonance mode can be kept in single mode even when the beam cross-section is increased, thereby realizing a high-output characteristic.
Further, Japanese Published Patent Application No. Hei. 11-233889 (the '889 application) discloses an electrode structure for uniformizing the carrier density in an active layer in a surface emitting laser.
In the surface emitting laser disclosed in the '889 application, distribution of current to be injected into the active layer is controllable by dividing a rear electrode into plural parts, thereby realizing a large-aperture surface emitting laser.
In the surface emitting laser 200 disclosed in the '889 application, the carrier density distribution in the active layer becomes a problem in realizing high-output characteristic. That is, in the surface emitting laser 200 disclosed in this document, the light intensity distribution in the region of the active layer 3 opposed to the lower electrode 600 is close to Gaussian distribution as shown in FIG. 14(c), and a peak of the light intensity Lp exists at the emission center of the active layer. On the other hand, in the region of the active layer 3 opposed to the lower electrode 600, the density Cd of carriers injected into the active layer 3 is uniform although the light intensity distribution significantly differs between the center portion of this region and the peripheral portion thereof. Therefore, in the peripheral portion of the center portion of the region of the active layer 3 opposed to the lower electrode 600, the density of carriers existing in the active layer is high, whereby an excess of carriers occurs. On the other hand, in the center portion of this region, shortage of carriers occurs. Such uneven distribution of carriers causes refractive-index distribution in the active layer 3, whereby the resonance mode undesirably becomes multimode. Further, there is a fear of gain saturation.
This phenomenon is more remarkable in a nitride-base semiconductor laser in which the threshold carrier density at which gain saturation occurs is extremely high and the differentiation gain is also high, than in an infrared semiconductor laser comprising AlGaAs system semiconductor material (AlxGa1-xAs (0≦=x≦1)) and a red semiconductor laser comprising AlGaInP system semiconductor material (AlxGayIn1-x-yP (0≦x≦1, 0≦y≦1)).
Further, in the surface emitting laser disclosed in the '889 application, loss occurs in the current injected into the active layer due to a resistive separation layer disposed in the part where the rear electrode is divided into plural parts, leading to a reduction in efficiency.